The present invention belongs to a technical field of an image forming apparatus which writes an electrostatic latent image onto an image carrier by writing electrodes of a writing device thereby to form an image and, particularly, to a technical field of an image forming apparatus which writes an electrostatic latent image onto an image carrier by charge injection between writing electrodes and the image carrier.
An image forming apparatus of which an image carrier is charged by injecting charge directly to the image carrier on which a latent image will be formed has been proposed by Japanese Unexamined Patent Publication No. H6-3921. The image forming apparatus disclosed in this publication has a charge injection layer on a photo-conductive layer of a photosensitive drum. A contact charging member is in contact with the charge injection layer to inject charge to the charge injection layer, thereby uniformly charging the photosensitive drum. The charge injection layer is formed by a binder resin composed of a phosphazene resin and a conductive filler of SnO2 dispersed in the binder resin so as to have a predetermined thickness.
As another conventional image forming apparatus, an image forming apparatus which employs electrodes as a writing device and writes an electrostatic latent image onto an image carrier by the electrodes has been proposed by Japanese Unexamined Patent Publication No. S59-33969. The image forming apparatus disclosed in this publication comprises a large number of pin electrodes, and a recording drum which is a metallic drum having a dielectric layer formed on the surface thereof. All pin electrodes are driven to make discharge phenomenon between the pin electrodes and the recording drum which are spaced apart from the other, thereby forming a solid black latent image for every line onto the surface of the recording drum.
As still another conventional image forming apparatus, an image forming apparatus which writes an electrostatic latent image onto a surface of a recording medium in the ion flow system as a writing device has been proposed in Japanese Unexamined Patent Publication No. H6-8510. The image forming apparatus disclosed in this publication comprises a corona charger and an aperture electrode which controls a flow of corona ions generated from wires of the corona charger. In the apparatus, an electrostatic latent image is formed on the surface of the recording medium by the controlled ion flow.
As for the image carrier disclosed in the aforementioned Japanese Unexamined Patent Publication H6-3921, the charge injection layer is formed in a wide range of the photo-conductive layer of the photosensitive drum and the conductive filler of SnO2 is dispersed in the binder resin. When the dispersed amount of SnO2 is too large, the surface resistivity of the charge injection layer should be too low, leading to drifts of latent image charge. On the other hand, when the dispersed amount of SnO2 is too small, the surface of the charge injection layer has poor exposure of SnO2, leading to poor injection of charge and thereby partially producing insufficient charged portions. Therefore, there are disadvantages that the lateral leakage of latent image charge can not be securely prevented, that the setting of dispersed amount of SnO2 is troublesome, that the stable charge is hardly achieved, and that the manufacturing of this image carrier is difficult.
On the other hand, in either of the image forming apparatuses disclosed in Japanese Unexamined Patent Publication No. S59-33969 and Japanese Unexamined Patent Publication No. H6-8510, writing is conducted by using discharge phenomenon so that the voltage to be applied should be very high. Since ion functions the role of the carrier, ionization due to the discharge phenomenon depends on the environmental conditions such as temperature and humidity. Variation in ionization may distort the positions of a latent image to be written. Therefore, there is a disadvantage that it is hardly stably charged.
The present invention was made in the light of the above described problems and the object of the present invention is to provide an image carrier which is capable of securely preventing the leakage of charge in lateral direction so as to stably conduct the application or removal of charge and which can be easily manufactured.
To solve the aforementioned problems, an image carrier of the present invention comprises a dielectric layer, wherein charge is transferred between said dielectric layer and a charge-transfer controlling means so as to apply charge to or remove charge from said dielectric layer, and is characterized in that said dielectric layer has a low-resistance layer formed on the outer surface thereof, said low-resistance layer comprises a large number of conductive portions, charge is transferred between said conductive portions and said charge-transfer controlling means so as to apply charge to or remove charge from said conductive portions, and said conductive portions are arranged to be dispersed separately from each other.
The image carrier of the present invention is further characterized in that said conductive portions are a large number of dots which are dispersedly arranged, that said large number of conductive portions are at least partially exposed on the surface of said low-resistance layer, that the electric resistance of said low-resistance layer is anisotropic in such a manner as to satisfy xe2x80x9cresistance in a direction perpendicular to the plane direction of said low-resistance layer (i.e. in vertical direction) less than resistance in the plane direction of said low-resistance layer (i.e. in lateral direction)xe2x80x9d, and that the thickness of said low-resistance layer is set to be 1 xcexcm or less.
According to the image carrier of the present invention, since the large number of conductive portions which are separately and dispersedly formed in the outer surface of the dielectric layer and the application or removal of charge can be conducted dominantly by charge injection between the conductive portions and the charge-transfer controlling means, the voltage to be applied can be significantly reduced as compared with the conventional device which applies or removes charge by discharge phenomenon.
Since a large number of the conductive portions are separately dispersed, charge applied to the conductive portions can be prevented from leaking in the lateral direction and charge on charged conductive portions can be prevented from leaking i.e. from moving to another conductive portion. Therefore, stable application or removal of charge relative to the image carrier can be conducted by charge injection.
Particularly, since the conductive portions are a large number of dots separately dispersed, the stable application or removal of charge can be conducted with higher precision. Further, the large number of conductive portions are partially exposed, thereby further reliably conducting the stable application or removal of charge relative to the image carrier.
Since the electric resistance of the low-resistance layer of the image carrier is set such that the resistance in the vertical direction is smaller than the resistance in the lateral direction, the leakage of charge in the lateral direction can be further securely prevented in the low-resistance layer so that charge can be effectively transferred between the charge-transfer controlling means and the low-resistance layer, thereby achieving the reliable application or removal of charge relative to the image carrier.
Since the thickness of the low-resistance layer is set to be 1 xcexcm or less, the electric resistance can be easily set such that the difference between the resistance in the lateral direction and the resistance in the vertical direction is enlarged by just forming the low-resistance layer to have a small thickness. Therefore, the potential contrast of the electrostatic latent image can be larger, thereby further improving the precision in writing latent images.
On the other hand, the method of manufacturing the image carrier of the present invention comprises previously forming a large number of concavities in the outer surface of the dielectric layer so that the concavities are dispersed separately from each other, coating conductive material onto the surface of the dielectric layer formed with the concavities, and then grinding the coated conductive material. According to this method, the large number of conductive portions separately dispersed can be easily formed. Therefore, the image carrier can be easily manufactured.
In another method of manufacturing the image carrier of the present invention, a liquid, prepared by dispensing conductive particles dispersed into the predetermined liquid, is displayed onto predetermined positions of the outer surface of an image carrier made of an insulating material which is soluble relative to the predetermined liquid, thereby forming the conductive portions. Also according to this method, the large number of conductive portions separately dispersed can be easily formed. Therefore, the image carrier can be easily manufactured.